Generally, the fusing unit for settling the toner images in a copy machine are used in many cases in the form of a heating roller type fusing unit. This heating roller type fusing unit is known to have high thermal efficiency, and have almost no danger of causing a fire to the paper when jamming. But the problem in this type of fusing unit is that it produces wrinkles on the copied papers. In particular this problem happens frequently in the case of copying the paper on both sides.
In the conventional heating roller type fusing unit as shown in FIG. 1, the wrinkle prevention measure is provided in such manner that the heating roller 1', coated with teflon and the like which can withstand the offset prevention agent, is formed in the shape of an inverse crown. That is, the diameter of the middle portion is slightly smaller than that of the end portions so that wrinkles can be prevented due to the lateral tension caused by the speed difference in conveying the paper. But in such a device, the copied paper produces wavy deformations on its outer edges due to the fact that the opposite side edge portions of the paper receive larger amounts of heat and pressure than the middle portion.
Therefore, if both sides of a paper are copied, the wavy deformations are further increased, forming visible wrinkles. Therefore, the provision of the inverse crown shape gives rather an adverse effect.
Prior attempts to solve this problem, for stretching the paper tightly before the paper enters into the nip portion of the fusing unit, include applying a bias voltage to the guide member, or two heating rollers having different inverse crown are provided in such manner that the moderate inverse crown roller is used for copying a paper on one face, and the steeper inverse crown roller is used for double face copyings. Another attempt includes slightly raising the middle portion of the guide member in such manner that the leading edge of the paper enters into the nip portion simultaneously.
However, all such attempts have not been successful in essentially removing the phenomenon of the wavy deformations and the wrinkles.